Podocyte dysfunction : an approach towards glomerular diseases with emphasis on nephrin

Abstract

Podocytes are terminally differentiated and structurally unique cells having interdigitating foot processes which form filtration slits. These slits are bridged by specialised podocyte cell junctions called slit diaphragms. Nephrin is an essential structural and signalling molecule within the slit diaphragm that plays a pivotal role in actin dynamics and podocyte junctional remodelling. Most glomerular diseases are caused by podocyte dysfunction, and glomerular disease progression directly correlates with podocyte numbers. Towards facilitating the quantitation of podocyte numbers, a convenient and robust methodology was developed applicable for high throughput usage. Progression was studied using hDTR and 4-EBP1 transgenic rat models. Progression non-invasively monitored through Nephrin / Podocin mRNA ratios correlated well with podocyte morphometrics. Nephrin mRNA down-regulated after acute injury was thought to facilitate podocyte detachment and contribute towards glomerular progression. An inducible Nephrin-KO mouse was generated to study the time-dependent development of FSGS, progression and podocyte loss. Molecular studies were conducted to understand Nephrin's role in foot process maintenance related to progression mechanisms. Podocyte remodelling involves actin dynamics regulated by Fyn phosphorylation and Nck adapter proteins that bind to Nephrin and recruit other downstream molecular players. In cell culture models, Nephrin actin dynamics involved lamellipodia regulation through ship2, filamin and lamellopodin. Integrin activation phosphorylated Nephrin through an inside signalling pathway. Shp2 deletion prevented Nephrin phosphorylation protecting mice from foot processes effacement. Endocytic trafficking of Nephrin was studied with Vps34 and PIKfyve KO models. Nephrin was trapped within Vps34-KO vesicles and mistargeted from its slit diaphragm position. Together, these studies have helped better understand glomerular disease progression and Nephrin's role in foot processes maintenance and progression. Future challenges and avenues studying Nephrin involve transcriptional regulation and molecular trafficking of Nephrin to the membrane